Richard Clark wrote in
:

On 25 Apr 2009 06:33:45 +0100, Gordon wrote:

I have been experimenting with fabricating a decent
indoor DTV antenna.

Hi Gordon,

This is a goal in conflict if we are to take it at face value. For
one, DTV has design demands that call for widebandedness. This, and
the combination for VHF coverage combine to drive the design to being
large. This is typically the domain of outdoor antennas, and for good
reason (large for DTV is going to be honestly large).

I know. I'm trying to find a good compromise. But it may not
exist. Unfortunatly, I can't mount an outdoor antenna. I rent,
and the landlord won't alow it.

I have tried several things. What I have
now is two bowties spaced about 7 inches apart on a wood dowel.
A peanut butter jar with rocks in it makes up the base.

It seems to give pretty good performance. It could be better.
But I am wondering about a few things.

1) Right now all the digital transmissions are in the UHF band.
but in 6 weeks, three stations will move back to their VHF
assignments in the VHF high band.
How well can I expect my antenna to work in that frequency band?

This is a simple matter of scaling. If the frequency shift is by an
order of, say, 2:1; then the size will increase by an order of 1:2.
Frequency and size are in an inverse relationship. Try doubling the
size of your current design, and looking for a gallon size peanut
butter jar.

I've done the math. Channel 2 has a wavelength of about 18 feet and
channel 50 is about 1.4 feet. More than a 2:1 ratio. I do catch a
break since the VHF lo band is going to be vacated in this area. So
channel 8 is the lowest frequency I need to worry about (wavelength
of approx 5.5 feet).


2) I have not trimmed or dressed the twin lead from the bowties.
I understand that twin-lead can act as antennas. So what
is the best way to manage the twinlead? Cut it back? I noticed
that when i laid the leads together, in an attempt to make a
neat assembly, I got signal cancelation.

This is an indoor antenna. It is going to suffer from any number of
things in proximity and your attempts to dress the leads probably were
not responsible for the signal quality change you observed. I will
bet that attempt to make a neat assembly had a lot of re-arranging
going on in addition to the lead dressing.


Actually, I tried to keep the situation as controlled as possible.
I initially had the leads just drapped down the "mast" (dowel) to
a balun. It was my wife that tried to improve the asthetics by
neatening up the twinleads. Then she complained that reception
was bad. That's when I was experimenting with the dressing of
the leads. Right now I have it "dressed ugly", but it works.

3) Any suggestions to improve this design?

4) Any suggestions for alternitive DIY designs?

Wait until you have a problem before you try solutions. As I said
above, your goal is in conflict with interior available dimensions.
The truth of the matter is that a simple rabbit ears antenna might do
just as well as the best of the DIY designs littering pages across the
WWW.

No, the rabbit ears didn't work very well at all. That's why I have been
experimenting. I have seen a lot of the DIY designs littering the
WWW. This dual bowtie design is mine, tho.


73's
Richard Clark, KB7QHC